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Originally published as Genetics Published Articles Ahead of Print on May 4, 2007.
Genetics, Vol. 176, 1403-1415, July 2007, Copyright © 2007
doi:10.1534/genetics.107.071621
Microhomology-Mediated End Joining in Fission Yeast Is Repressed by Pku70 and Relies on Genes Involved in Homologous Recombination
Anabelle Decottignies1
Cellular Genetics, Christian de Duve Institute of Cellular Pathology, Faculty of Medicine, Catholic University of Louvain, 1200 Brussels, Belgium
1 Address for correspondence: ICP, Faculty of Medicine, Catholic University of Louvain, Avenue Hippocrate, 74+3, 1200 Brussels, Belgium.
E-mail: anabelle.decottignies{at}gece.ucl.ac.be
Two DNA repair pathways are known to mediate DNA double-strand-break (DSB) repair: homologous recombination (HR) and nonhomologous end joining (NHEJ). In addition, a nonconservative backup pathway showing extensive nucleotide loss and relying on microhomologies at repair junctions was identified in NHEJ-deficient cells from a variety of organisms and found to be involved in chromosomal translocations. Here, an extrachromosomal assay was used to characterize this microhomology-mediated end-joining (MMEJ) mechanism in fission yeast. MMEJ was found to require at least five homologous nucleotides and its efficiency was decreased by the presence of nonhomologous nucleotides either within the overlapping sequences or at DSB ends. Exo1 exonuclease and Rad22, a Rad52 homolog, were required for repair, suggesting that MMEJ is related to the single-strand-annealing (SSA) pathway of HR. In addition, MMEJ-dependent repair of DSBs with discontinuous microhomologies was strictly dependent on Pol4, a PolX DNA polymerase. Although not strictly required, Msh2 and Pms1 mismatch repair proteins affected the pattern of MMEJ repair. Strikingly, Pku70 inhibited MMEJ and increased the minimal homology length required for efficient MMEJ. Overall, this study strongly suggests that MMEJ does not define a distinct DSB repair mechanism but reflects "micro-SSA."
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